I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method for providing both code division multiple access and time division multiple access wireless telecommunications service in an area having a Global System for Mobile (GSM) communications network.
II. Description of the Related Art
Code division multiple access (CDMA) wireless telecommunications is the next generation of digital wireless telecommunications. CDMA allows wireless telecommunications systems such as cellular telephone systems to provide higher quality wireless telephone service at a reduced cost by more efficiently using the radio frequency (RF) bandwidth available to conduct telephone calls. Additionally, CDMA provides a more reliable, fade-free, RF signal interface than previously existing wireless communication systems and methods. The leading standard for CDMA wireless telecommunications is the IS-95 over-the-air CDMA interface standard, promulgated by the Telecommunications Industry Association (TIA). In addition to the general advantages provided by CDMA noted above, IS-95 CDMA provides soft handoff capability by allowing a subscriber unit (usually a cellular telephone) to engage in multiple RF interfaces simultaneously. Soft handoff increases the likelihood that a subscriber unit will remain connected as the set of base stations with which it interfaces changes in response to changing location.
To allow the widest possible availability of CDMA wireless telecommunication service, it is desirable to reduce the cost associated with the implementation of a CDMA wireless telecommunications system. One such method and system is described in copending U.S. patent application disclosure no. 08/575,413 entitled "WIRELESS TELECOMMUNICATIONS SYSTEM UTILIZING CDMA RADIO FREQUENCY SIGNAL MODULATION IN CONJUNCTION WITH THE GSM A-INTERFACE TELECOMMUNICATIONS NETWORK PROTOCOL" filed Dec. 20, 1995 and assigned to the assignee of the present invention. (CDMA-GSM network application) In the CDMA-GSM network application a wireless telecommunications system using an IS-95, or IS-95 like, over-the-air interface in conjunction with a Global System for Mobile communication (GSM) telecommunications network is described. GSM telecommunications systems represent the previous generation of digital wireless telecommunication systems and utilize a time division multiple access technology over the air interface (GSM-TDMA). Such a wireless telecommunications system allows CDMA telecommunications service to be introduced at a reduced cost in areas having already existing GSM base telecommunication service, because a substantial portion of the previously existing infrastructure can be used to provide the CDMA wireless telecommunication service.
Allowing a CDMA based wireless telecommunications system to be built using preexisting telecommunications infrastructure also raises the possibility of CDMA and GSM-TDMA, or CDMA and other prior art wireless telecommunication communication systems, being simultaneously implemented within the same region or area. Having both CDMA wireless telecommunications service and the preexisting telecommunication service available in the same area has various advantages. These include including the ability to incrementally introduce CDMA service within a service area thereby spreading out the cost of introducing CDMA service over time, as well as the ability to perform limited introductions of CDMA service only in areas where demand for wireless telecommunication service has outstripped the supply provided using the prior art technology. In order to provide total mobility to CDMA service subscribers located in a areas where CDMA and prior art systems coexist, however, a system and method for allowing the subscriber unit to switch between the two types of service during a telephone call is necessary. Since mobility is one of the substantial benefits provided by wireless telecommunications service, such a system and method allowing a subscriber unit to switch between CDMA and another type of cellular telephone service would be highly desirable.